Beethoven himself gave a title to String Quartet No.15 in A minor, Op.132 - 3:
A Convalescent's Holy Song of Thanksgiving to the Divinity, in the Lydian Mode
From the language and context, and also the particular mention of the mode and not just the key, it is clear that Beethoven's usage of the
in the Lydian Mode was not just as a technical boiler-plate title, such as "Symphony in C Minor".
Why did Beethoven include "in the Lydian Mode" in this special title?
A Convalescent's Holy Song of Thanksgiving to the Divinity seems quite sufficient.